156 CHEMICAL COMPOSITION OF THE SOIL [chap. 



brought about by lack of phosphoric acid or lack of 

 potash, because these substances affect the whole course 

 of development of the plant ; yet when the apples grown 

 on a particular field are always very red or the wheat is 

 very strong, it is useless to expect to find some substance 

 in the soil to which the redness or the strength is due. 

 It seems so straightforward to analyse the "strong" 

 wheat and ascertain the substance which causes the 

 strength, and then proceed to find the same substance 

 in the soil giving rise to strength. Unfortunately, 

 things do not happen in this simple way : the differences 

 in quality are usually due to very subtle differences in 

 the construction of the plants' constituents, and not in 

 the materials — carbon, hydrogen, nitrogen, oxygen, etc. 

 — out of which the structures are made. In nearly all 

 cases, also, the structure is mainly determined not by the 

 plant food or the manure available, but by the climate, the 

 water - supply, the temperature, and other physical 

 conditions of the soil. Just in the same way the old 

 theory must be erroneous which explained the value of 

 a rotation of crops by supposing that each plant in 

 turn extracted one particular substance from the soil, 

 and so temporarily exhausted the land for itself but 

 not for other crops requiring a different nutrient. 

 We see that no single crop can exhaust the soil of any 

 of its essential constituents, we see also that all crops 

 take out very much the same elements and in similar 

 quantities — therefore the value of a rotation depends 

 on a quite different set of factors. Putting aside the 

 economic arrangement of the labour which a rotation 

 permits, its great value lies in the facility it affords for 

 cleaning the land without any special labour. When- 

 ever one crop is grown continuously on the same land, 

 as is done on the Rothamsted experimental plots, 

 certain weeds which are favoured by the particular crop 



